(1) Field of the Invention
This invention relates to a method for polymerizing technical lignins. More particularly, this invention relates to a sequential treatment with formaldehyde and air (or molecular oxygen) to increase the molecular weights of said lignins.
(2) The Prior Art
Lignin, as it occurs in all woody plants, is a three-dimensional macromolecule consisting of rather hydrophobic phenylpropane units. Because of its structure and high molecular weight, lignin imparts rigidity to cell walls of woody tissues and gives an outstanding resistance of trees towards impact, bending and compression. In pulping processes for manufacture of wood pulps, lignin is chemically depolymerized and solubilized in pulping liquors to facilitate its removal from wood fibers. As a consequence, the molecular weights of technical lignins isolated from the pulping liquors are polydisperse and relatively low in comparison with that of native lignin in wood. For instance, there are many reports of molecular weights below 1,000; and values greater than 1,000,000 have been published for both lignosulfonates and alkali lignins. However, the average molecular weights are much lower than the reported maximum value, and range from 3,000 to 4,000 for pine or hardwood kraft lignin, and from 300 to 20,000 for spruce lignosulfonate.
That technical lignins, both alkali lignins and lignosulfonates, possess relatively low molecular weights, as compared with many commercially available synthetic polymers, is a significant limitation which hinders the use of said lignins in many application areas. For this reason, numerous methods are revealed in the prior art for polymerization of said lignins to increase their molecular weights. Thus, in U.S. Pat. No. 3,138,555 granted to King and Adolphson, an acid polymerization process is revealed for condensation of lignosulfonate components of spent sulfite liquor-derived solids, under controlled conditions, thereby increasing the effectiveness of lignosulfonates as dispersants in clay-water syspensions, such as drilling muds. In U.S. Pat. No. 3,864,276 to Benko and Daneault, useful dispersing agents are produced from mixtures of spent sulfite liquor solids and kraft liquor solids by treating said mixtures with air or formaldehyde as a cross-linking agent.
Brauns, The Chemistry of Lignin, 546-549, 1952, and Brauns and Brauns, The Chemistry of Lignin, Supplement Volume, 513-515, 1960, discuss the oxidation of lignin with oxygen or air in an alkaline medium. Generally, the oxidation of lignin disclosed in the prior art cited therein is aimed at degradation of the lignins to low molecular weight compounds, rather than condensation to high molecular weight compounds.
In U.S. Pat. No. 3,857,830, Briggs discloses a process for cross-linking lignin with a di-epoxide to increase its surface active properties and thus creating an improved flocculating agent. The process specifies as a cross-linking agent the use of di-epoxide having a molecular weight in the range of from 120 to 1,800 and to the extent of cross-linking that the phenolic content of the lignin has been decreased by from about 40% to 95%. Ludwig, in U.S. Pat. No. 3,850,799, describes a process whereby lignin is treated with a polyoxyalkylene di-ester of a monosulfonic acid or a polyoxyalkylene dihalide to obtain high molecular weight compositions which are effective as thickening and flocculating agents.
In those use areas wherein lignin is incorporated into elastomers such as rubbers, to achieve reinforcement effect, lignin has been invariably treated in a manner that the thermal properties of lignin are improved substantially. The increase in heat resistance of lignin resulting from the reaction with formaldehyde is taught by Ball in U.S. Pat. No. 3,312,643. In U.S. Pat. No. 2,610,954, Raff and Tomlinson disclose a treatment of lignin with hot air whereby the melting point of alkali lignin is increased from less than 220.degree. C. to over 300.degree. C., improving the reinforcing properties of said lignin, when incorporated into natural and/or synthetic rubbers. It is known now that mild oxidation with air is an effective means for increasing the molecular weight of lignin.
In terms of reactive sites of lignin wherein a cross-linking reaction occurs, methods for polymerizing the lignin may be classified into two types:
(1) cross-linking or bridging of lignin molecules by reacting free phenolic hydroxyl groups with bifunctional chemicals (hereafter termed phenolic polymerization); and
(2) condensing of lignin units at other non-phenolic sites (hereafter termed non-phenolic polymerization).
In phenolic polymerization, lignin units are cross-linked by blocking the free phenolic hydroxyl groups with halogen- or epoxide-containing agents such as epichlorohydrin, di-epoxide and polyoxyalkylene dihalide. This type of polymerization reaction obviously shows three disadvantages:
(1) It requires relatively expensive cross-linking chemicals;
(2) blocking of phenolic hydroxyl groups reduces the solubility of lignin in alkaline aqueous systems and thus greatly increases the viscosity of reaction media; and
(3) lignin becomes inactivated in many modification reactions which need free phenolic hydroxyl groups.
On the other hand, non-phenolic polymerization of lignin does not render free phenolic hydroxyl groups unavailable for further reactions; and it yields a polymerized lignin soluble in alkaline media. The most useful non-phenolic polymerization processes of lignin include treatments with formaldehyde or air as already mentioned in above-cited patents. The mechanisms of these treatments in condensing lignin structures have been well elucidated in the prior art. A treatment of lignin with formaldehyde gives rise to the formation of biphenyl methylene linkages, and mild oxidation with air (or molecular oxygen) generates free radical intermediates which combine to form a biphenyl structure. Both biphenyl methylene linkage and biphenyl structure are responsible for increase in molecular weights of lignin. It should be pointed out that the cross-linking mechanisms in formaldehyde treatment or air oxidation require free ortho position (to phenolic hydroxyl groups) in a lignin aromatic unit. Consequently, these non-phenolic polymerization methods are not effective for increasing the molecular weight of hardwood technical lignins which contain a large number of syringyl units.
It is accordingly a principal object of this invention to provide an economical method for obtaining high molecular weight technical lignins.
It is an additional object of this invention to provide a method for polymerization of lignin using formaldehyde and air as the cross-linking agents.
It is still an additional object of this invention to provide a simple process for condensing hardwood lignins.
A further object of this invention is to provide a method for polymerizing lignins whereby free phenolic hydroxyl groups of lignin are not blocked.
Other objects, features and advantages of the invention will be clear from the following detailed description of the invention.